Intermittent, positive pressure breathing systems represent an important facet in medical respiratory therapy. In general, these systems respond to inspiratory impulses to deliver quantities of oxygen and/or other supportive gases to a patient under a mild positive pressure. Gas flow communication between the patient and the system is by a mask or mouthpiece coupled through tubing to a somewhat elaborate control apparatus serving to regulate gas flow, pressure and the like. Among the advantages of its use, the therapy has been described as serving to increase tidal volume and total minute ventilation; to decrease the work or "cost" of breathing, to facilitate the elimination of carbon dioxide by increasing alveolar ventilation; to increase arterial and tissue oxygen tension; to provide mechanical bronchodilation, and to prevent or correct atelectasis.
To further expand the therapeutic capability of the systems, the common practice has been to generate medicament carrying aerosols within the treatment mechanism conducting oxygen to the patient. Thus incorporated, the system retains an advantageous capability of delivering all medications used in aerosol therapy, i.e. bronchodilators, mucolytics, detergents, antibiotics, proteolytics, anti-foaming agents and wetting agents. For optimum performance in carrying out aerosol therapy it has been determined that the particle or droplet size distribution of the aerosol mist should be optimized to achieve requisite introduction of medicament into the lung. For example the lung is structured having twenty-three generations of subdivisions, the trachea being the zeroeth generation, the left and right main branches forming the first generation, etc. Should the size of the medicament carrying aerosol particles be too large, it is opined that they would be impacted upon the oral pharynx and not reach the bronchial tract, thus rendering the therapy substantially ineffective. On the other hand, some authority suggests that where the aerosol particles are too fine, i.e. of too small an average diameter, significant portions of the mist may not be absorbed but will be exhaled. Accordingly, a most desired medicament carrying aerosol development is one wherein an optimized distribution of droplet or particle size is achieved. For example, it is desirable that the particle size distribution be somewhat monodisperse, the greater number of particles having a diameter of about 2 microns. Particles of that diametric extent would be suited for a deposition throughout a sufficient number of generations of the lung extending toward the alveoli.
Considerations of efficient clinical practice require that the nebulizer devices which generate the medicament carrying aerosols or "micro mists" be fabricable at cost levels such that both the nebulizer and connecting flexible tubing, mouthpieces and the like leading from more complex control equipment to the patient be disposable after one use. This feature of disposability permits a clinical assurance of establishing requisite sterility for all components and as a consequence, minimal opportunity for human error to occur.
Nebulizer devices currently used or proposed generally fail to meet all of the above-discussed criteria. For instance, disposable devices typically in clinical use form the aerosol mist by introducing a stream of medicament carrying liquid into the gas stream leading to the mouthpiece or mask. Sometimes identified as the "Bernoulli Effect", a jet of gas interacts with the liquid stream to shear off droplets. Conventionally some form of spherical target is positioned downstream from the point of liquid introduction to achieve a break up of the larger of these air entrained droplets. Generally, these devices evolve a particulate distribution of the aerosol having a relatively large number of particles of larger diameter and mass. These larger particles have only marginal therapeutic effect upon reaching the lung. Ultrasonic devices have been proposed but are regarded as too expensive to manufacture for requisite disposability and, additionally, for typical application, these devices tend to produce too dense a cloud of aerosol for practical utilization within the tubular conduits typically provided, in conjunction with lung ventilating equipment.